Deep inside a mountain, a beam of tiny particles flies into a cloud of gas. This is how scientists recreate the reactions that happen inside stars. Previously, a thin film, like a pane of glass, stood between the beam and the gas. It slowed down the particles and made it hard to measure exactly what was happening. Now the window is gone, but the gas doesn't scatter: a system of pumps holds it in place, much like water circulates in a fountain.
The beam heats the gas, causing it to expand, like air above hot asphalt. If you don't account for that, your measurements will be off. So the scientists built two instruments. One, like a scale, measures the heat from the beam. The other, like a finger tracing a thread, checks the gas thickness along the entire path of the particles.
The window that used to block the view has been removed. Now we can see the very flame of stellar reactions.
The calorimeter measures beam power to within tenths of a watt, even though the beam itself carries hundreds of watts. That is like noticing that a kettle has cooled by half a degree when you add a spoonful of cold water.